Coaxial cable isolators, for providing an interruption or gap in the ground conductor between a user apparatus and an incoming cable ground, are well known in the art. Specifically, U.S. Pat. No. 4,399,419 to P. Dobrovolny and assigned to the assignee of the present invention includes a plurality of disc-shaped ceramic capacitors and ferrites. The capacitive elements had values selected to enable coupling of television signals while blocking direct current and 120 V 60 Hz low frequency power currents. Since the outer ground conductor is interrupted, the isolator is subject to electromagnetic interference (EMI) and the ferrites are used to absorb any such energy that enters the interruption. Coaxial cable isolators of this type have been well received.
The value of the capacitance exhibited by the capacitive elements should be as large as possible to maximize television signal coupling and yet not be so large as to pose safety problems. Also, the isolator ideally should be physically small to enable it to fit into a device, such as an amplifier, and should also exhibit a small capacitance to limit signal leakage, obviously somewhat incompatible requirements. High dielectric constant capacitors, such as those produced from titanate ceramics, have proven useful for such applications. A serious drawback is that the higher the dielectric constant of the titanate formulation, the greater the capacitance temperature coefficient and hence the greater the change in exhibited capacitance with temperature change and the more nonlinear the temperature coefficient is. There has long been a need to obtain a high dielectric constant ceramic formulation that has a minimum temperature dependence. The temperature coefficient of the ceramic is mainly a function of formulation, but is also dependent upon firing temperature.
In the United Kingdom a maximum limit of 5 nonofarads is specified for a 75 ohm coaxial cable isolator. It will be recalled that in the United Kingdom, as in many foreign countries, residential power is supplied at 240 volts rather than at 120 volts as in the United States. Consequently, the capacitance value of the isolator should be lower for safety reasons. Yet, for effective EMI attenuation, the lower limit of capacitance is about 3.8 nanofarad. When the range of environmental temperature to which the isolator is subject, and the large nonlinear temperature coefficient of a high dielectric constant material are considered, many problems are manifest. For a normal outdoor temperature of -15 degrees Centigrade to 40 degrees Centigrade, the capacitance presented by high dielectric constant materials is often too low to provide adequate signal coupling. On the other hand, adding capacitive elements results in too much capacitance at certain temperatures within the operating temperature range of the isolator.
In U.S. Pat. No. 3,549,415 issued to R. Capek and J. Mazintas and assigned to the assignee of the present invention a method of making a multilayer ceramic capacitor is described. In that patent a capacitor includes a plurality of ceramic layers or wafers separated by conductive plates that are alternately connected together to form end leads. The dielectric materials of the wafers are individually selected, based upon their temperature coefficients, to produce a more linear temperature coefficient for the capacitor. That invention involved calcining the thin, flat wafers before sintering to prevent material diffusion between wafers. Diffusion during sintering apparently generated chemical-like reactions that caused substantially different solid solutions, in which the temperature coefficient of the resulting ceramic was no longer related to the ingredient's proportions. The separate precalcining of the ferroelectric wafers precluded diffusion between the layers during sintering.
Thus, the concept of making a multilayer capacitor comprising dielectric materials of different temperature coefficient to achieve a more linear composite temperature coefficient for the capacitor is known. The patented multilayer capacitor was approximately 0.75 inch by 0.1 inch by 0.5 inch and included 6 to 12 layers, with each layer varying from about 0.002 inch to 0.015 inch. An average layer thickness, for low voltage circuits, was about 0.006 inch. The specified temperature range of 25 degrees Centigrade to 120 degrees Centigrade is significantly higher than that under present consideration and no indication of making disc-shaped or tubular capacitors is given.
This prior art method of making a temperature compensated capacitor is difficult and is generally inadequate for use in connection with a tubular coaxial cable isolator, especially one that is subjected to large working voltages, such as 240 volts and to test voltages 1180 volts AC or 2180 volts DC.